In fibre-optic endoscopes used in laparoscopy, a lens focuses an image of the object on the distal ends of a coherent bundle of optical imaging fibres. The image formed at the proximal end of the optical imaging fibres can be formed by a suitable lens into a real image for direct viewing, or can be focussed onto the image sensor of a video camera. The imaging bundle is surrounded by a layer of illuminating fibres through which light from a suitable high-intensity source is conducted to the distal end of the endoscope to illuminate the object.
Known video-based fibre-optic imaging systems are usually assembled from standard, commercially-available components: the image from the imaging bundle is focussed on the image sensor of a color video camera, and the resulting video signal is displayed on a commercial color video monitor. The illuminating fibres are normally illuminated with light generated by a 300-Watt Xenon-arc a 150-300-Watt metal halide light source, or some other suitable light source. Video cameras used in known video-based imaging systems use systems developed for the consumer and industrial video markets to control parameters affecting image quality. The average luminance level of the video signal is controlled by an automatic shutter that examines the whole of each frame of the video signal and electronically adjusts the amount of light collected by the image sensor by altering the integration time of the sensor. White balance (color mix and balance) is determined for the whole frame, and is electronically corrected.
Some known high-intensity light sources include a servo-controlled shuttering system that changes the intensity of the light illuminating the illuminating fibres to control the average luminance level of the video signal generated by the camera. The servo systems in such light sources operate in response to the average luminance level of each frame of the video signal generated by the camera. The video signal generated by the camera is connected to a servo input on the light source; the average luminance level of this signal is proportional to the average light level impinging upon the whole of the image sensor in the camera. A motorized shutter screen operates in response to the video signal from the camera to change the intensity of the light illuminating the illuminating fibres so as to maintain the average luminance level of the video signal substantially constant. However, many current video-based fibre-optic imaging systems rely upon the electronic shutter in the camera to set the luminance level of the video signal generated by the camera, and the current trend is towards increasing use of this technique.
Most currently-available video-based fibre-optic imaging systems are optimized for large-diameter endoscopes having an outside diameter in the range of 5 to 10 mm (0.2" to 0.4") and using standard rod lens assemblies. Endoscopes having a considerably smaller outside diameter in the range of 1 to 2 mm (0.04" to 0.08") using Gradient Index (GRIN) lenses and fibre-optic imaging bundles have been developed and are also available for surgical applications. Such endoscopes are advantageous in that they further reduce the size of incision required to insert them into a body cavity.
While some known video imaging systems are capable of generating images from small diameter endoscopes, they are typically restricted to use at short working distances, typically less than 2" (50 mm). If the image from the fibre-optic assembly is formed on the image sensor in the camera so that the image covers the whole area of the sensor, the resulting video picture of an object at an extended working distance has insufficient intensity when normal levels of illumination are used. Moreover, the video picture of an object at any working distance is pixellated, i.e., the picture clearly shows the outlines of the individual optical fibres of the imaging bundle and the voids around them, if present. These shortcomings are a result of the small diameter of the imaging bundle, and the relatively few (typically 1,600 to 25,000) optical fibres in the imaging bundle of a small-diameter endoscope.
A more acceptable video picture is obtained by reducing the size of the image of the imaging bundle formed on the image sensor in the camera so that the image occupies a fraction of the area of the sensor. This arrangement produces a video frame in which a central image of the imaging bundle is surrounded by a blank external area, and results in a video picture in which the intensity of the image is increased and the pixellation of the image is reduced. However, this arrangement also has some disadvantages. The pixels of the image sensor in the external area surrounding the image generate noise, especially when the light level of the image is low. This noise is visible in the blank external area of the frame, and can be distracting to the observer.
Also, the video signal generated in response to a frame in which a central image is surrounded by an external blank area causes the above-mentioned image quality control systems in known imaging systems to operate non-optimally. For example, the control signal in the camera or the light source that controls the average luminance level of the video signal generated by the camera is derived in response to each complete frame of the video signal. A change in the intensity of an image occupying a fraction of the frame produces a smaller change in the control signal than a similar change in the intensity of an image filling the whole of the frame. As a result, the intensity control system cannot maintain the image part of the video picture at a substantially constant intensity. As a side effect of this, luminance saturation in the image can occur.
As another example, none of the image of the imaging bundle extends beyond the frame of the video picture. Consequently, any non-uniform illumination of the object, or varying radial sensitivity of the imaging bundle will be visible in the displayed image.
Small-diameter fiber-optic endoscopes present additional problems when used in large body cavities. In such applications, endoscopes with a hyper-extended working distance, greater than 50 mm (2) are used. Since the light reaching the image sensor is inversely related to the fourth power of the working distance, to provide the light level required to produce a satisfactory video image, a very high light intensity is required. With such a high light intensity, the distal tip of the endoscope can overheat if it comes into contact with tissue or objects.
Known camera-based or light source-based average luminance level control systems suffer from an additional operational problem as a result of their being responsive to the whole of the video frame. High intensity light reflected from highly reflective objects, such as metal instruments, fluid pools, etc., in the field of view can cause the control system to maintain the set average luminance level by reducing the luminance level of the whole of the frame. This can cause the part of the frame being observed by the surgeon to disappear into black level. Time must then be wasted repositioning the reflective object and/or the endoscope, or adjusting the video system, to restore the visibility of the part of the frame being observed.
It is known in consumer video systems to derive a signal for operating an auto focus system from an small area, normally in the center, of the image sensor in the camera. It is also known in endoscopic video systems to derive a signal for adjusting white balance from a small area, normally in the center, of the image sensor in the camera. In both of these known systems, however, the relationship between the small area and the image is undefined.